1. Field of the Invention
Performance additives in high performance polymers using polyhedral oligomeric silsesquioxanes (POSS) and polyhedral oligomeric silicates (POS) as nanoscopic reinforcements, porosity control agents, thermal and oxidative stability aids to improve the properties of the polymers.
2. Description of the Prior Art
There is a continuing need for polymeric materials that exhibit higher performance characteristics. In particular, many electronic and space vehicle component designs now demand materials with improved thermal and oxidative stability relative to that offered by the current level of imide, epoxy, and ester-based polymer resins. There exists a particular deficiency in the area of space resistant polymeric materials as there are no commercially available polyimides that are resistant to degradation by atomic oxygen. Prior art in this field describes attempts to improve survivability of imides to the space environment through the application of metals or metal-oxide coatings, which results in modest improvements but is not practical because of the additional processing steps and property mismatches (e.g. thermal expansion). Other approaches have involved the incorporation of fillers into polyimides through sol-gel methods or the blending of inorganic fillers. While conceptually simple the utility of this approach has also been limited. For example Yanno et al. have reported the use of complex processing steps (Hsiue, G -H, Chen, J -K., Liu, Y -L J. Appl. Polym. Sci., 2000, 76, 1609-1618) and Gilman et al. have described the inherent incompatibility of such organofunctionalize fillers to uniformally disperse (Brown, J. M., Curliss, D., Vaia, R. A., Chem. Mater., 2000, 12, 2279-3384) into the material at the molecular or nanoscopic level. Additional prior art has focused on the polymerization of silicones (Katz, U.S. Pat. No. 5,073,607) and phosphine oxides ((a) Smith, C. D., Grubbs, H., Webster, H. F. Gungor, A., Wightman, J. P., McGrath, J. E., 1991, High Perform. Polym, 3, 211. (b) Fewell, L. L., J. Appl. Polym. Sci., 1990, 41, 391) into polyimides in attempts to ensure uniform dispersion of an oxide forming component that can serve to protect the polyimide through formation of a passivating layer. This approach has been successful in retarding the rate at which damage in polyimides occurs during atomic oxygen exposure but the method has proven of little utility in protecting from degradation by other types of radiation nor is the approach general enough to offer protection to other types of polymeric materials, such as epoxies, esters, elastomers and sealants, that are also desirable for use on space vehicles.
A related need for higher performance polymeric materials also exists in many electronic component designs. The requirements for improved thermal (in excess of 400xc2x0 C.) and oxidative stability (to atomic oxygen, ozone, etc.) and reduced dielectric properties are similar to those needed for survivability in space environments. Prior art has been deficient in offering a generally applicable and easily implemented solution for upgrading the properties of imides, epoxy, ester and related polymeric materials desirable for use in the manufacture and packaging of electronic devices and systems. There exists a particular deficiency in the area of thermally stable, tough, and low dielectric constant (k less than 2.5) polymeric materials. Prior art in this field has involved the incorporation of fillers into polyimides through sol-gel methods or the blending of inorganic fillers. While conceptually simple the utility of this approach has also been limited due to inherent incompatibility, dispersion, and complex processing issues. Other approaches describe attempts to create desirable improvements in such polymers through the blending of amic-acid or imidized polymers with porogenic-type materials that introduce open-cell porosity upon their removal of the porgen by heating or extraction. (U.S. Pats. No. 6,204,202; 6,177,360; 6,107,357; 5,953,627). The effectiveness of this approach has been limited in that the introduction of open-cell porosity results in materials with poor ductility and durability whereas pores with a closed-cell structure would result in materials with more desirable properties.
All of the prior art pertaining to high performance polymeric materials fails to utilize nanoscopic entities as building blocks for the improvement of the characteristics of material and physical properties such as operational temperature range, durability, oxidative stability, flammability, and mechanical strength. Furthermore the prior art fails to recognize the important contribution that nanoscale reinforcements and varied nanoscopic topologies (shapes) can have on the physical properties.
Polyhedral oligomeric silsesquioxane (POSS) cage molecules, monomers, polymers, and resins as well as polyhedral oligomeric silicate (POS) (spherosilicate) cage molecules, monomers, polymers, and resins are increasingly being utilized as building blocks for the preparation of novel catalytic materials and as performance enhancement additives for commodity and engineering polymers. Their nanometer size and unique hybrid (inorganic-organic) chemical composition are responsible for the many desirable property enhancements that have been observed upon incorporation of POSS/POS reagents into polymer systems. Of special importance for high performance polymers is that the thermochemical properties of POSS molecules are very high (400-500xc2x0 C.). (Mantz, R. A., Jones, P. F., Chaffee, K. P., Lichtenhan, J. D., Gilman, J. W., Ismail, I. M. K., Burmeister, M. J. Chem. Mater., 1996, 8, 1250-1259) Additionally, POSS-siloxane copolymers have previously been shown to exhibit excellent resistance to oxidation by atomic oxygen. (Gilman, J. W., Schlitzer, D. S., Lichtenhan, J. D., J. Applied Poly. Sci. 1996, 60, 591-596). The ability of the nanoscopic POSS entity to be polymerized into all elastomers, thermoplastics, and thermoset polymers along with its inherent ability to absorb radiation and ability to form passivating silica layers upon oxidation renders it a general solution from which to develop the next generation of high performance resins for electronic and space system applications. The resulting silica layer and POSS nanoreinforcement also serve to protect the virgin material from damage by ultraviolet radiation as they both absorb UV of 256 nm and higher (FIG. 1).
This invention teaches the use of nanoscale POSS and POS chemicals as performance additives that can be polymerized or noncovalently blended into high performance polymers (imides, epoxies, ester) for the introduction of nanoscopic reinforcements, porosity control agents, thermal and oxidative stability aids that improve the interfacial, surface, physical and mechanical properties of high performance polymeric resin systems. The precisely defined nanoscopic features provided by the POSS/POS agents provide multi-length scale levels of reinforcement in such polymers and hence can be used synergistically with conventional fillers and fiberous reinforcements and fillers. POSS/POS can be incorporated into high performance polymers using nonreactive compounding or blending, reactive processing and reactive grafting, or through copolymerization.